The present invention relates to a magnetic disk drive that implements positioning of a magnetic head using a voice coil motor (abbreviated to VCM hereinafter), and to a speed control method using the back electromotive force of a VCM.
As disclosed in JP-A 11-25625, a VCM back electromotive force sensor is used for speed control during loading and unloading of the magnetic head in a magnetic disk drive system that has a mechanism that moves the magnetic head into an evacuation area on the disk surface when it is not operating, and this is called a load-unload system.
In a well-known method of detecting back electromotive force, a voltage that is proportional to the VCM drive current is subtracted from the VCM coil terminal voltage, as shown in FIG. 2. In this figure, the VCM coil 1 can be modelled, as shown in the diagram, as a series connection of three elements: resistor 4 of resistance value Rm; an inductance 5 of inductance value Lm; and a back electromotive force 6 that should be detected with those. The VCM coil 1 is connected in series with a current sensing resistor 3 to a driver circuit, not shown, and the current 2 is controlled by the driver circuit. In such a circuit, the resistance R1 of resistor 27 and the resistance R2 of resistor 28, which are connected to the operational amplifier 26, take on values that satisfy the relationship Rm=R2/R1xc3x97Rs. When the ratio between resistance R3 and resistance R4 of resistors 30 through 33 connected to operational amplifier 29 is G=R4/R3, the detected back electromotive force signal Bemf_h can be expressed by the following equation:
Bemfxe2x80x94h=Gxc2x7(Bemf+sLmxc2x7Im)+Vrefxe2x80x83xe2x80x83(Equation 1)
The principle of measurement according to Equation 1, which is associated with conventional back electromotive force sensors, will be explained in further detail using the block diagram shown in FIG. 9. As explained above, the VCM coil can be modelled by connecting, in series, resistor Rm, inductance Lm and the back electromotive force Bemf, that should be detected. The impedance Z of this VCM coil can be expressed as Rm sLm. Here, s is a differential operator. Block 112 shows the coefficient for conversion from [VCM coil terminal voltage Vs] to [VCM coil current Im], that is, the inverse of the VCM coil impedance. Reference numeral 104 denotes the VCM coil current Im. This current Im can be expressed by multiplying the inverse of the VCM coil impedance and the differential voltage 103 between the VCM terminal voltage Vs 101 and the back electromotive force Bemf 102 produced by movement of the actuator.
Back electromotive force sensors that use conventional technology calculate the voltage drop Vrm 105, caused by VCM coil resistance Rm, from the VCM coil current Im 104. They then determine the back electromotive force signal Bemf13 h 106 by subtracting this voltage drop from the VCM terminal voltage 101. In FIG. 9, block 113 represents the coefficient for conversion from [VCM coil current Im] to [current drop Vrm caused by VCM coil resistance], that is the VCM coil resistance Rm.
The following Equation 2 is derived from the above-described theoretical flow.
Bemfxe2x80x94h=Vsxe2x88x92Rmxc2x7Im=[Bemf+(Rm+sLm)xc2x7Im]xe2x88x92Rmxc2x7Im=Bemf+sLmxc2x7Imxe2x80x83xe2x80x83(Equation 2)
Equation 1 above is obtained when this theoretical equation is made to correspond to the circuit of the conventional back electromotive force sensor shown in FIG. 2.
Thus, the value of the back electromotive force detected in conventional circuits includes an item that is proportional to the VCM coil current Im differential. In other words, the second item sLmxc2x7Im in the parentheses on the right side of Equation 1 includes the differential operator s, which indicates a Laplace transformation. This item is proportional to the VCM coil current Im differential. Accordingly, when the VCM coil current Im changes and directly after it changes, the transient response caused by this differential item means that the correct back electromotive force cannot be detected. Therefore, in a load-unload control system that uses this back electromotive force sensor, the transient response caused by this differential item is set by using a sufficiently large control sample period (for example 700 micro seconds (xcexcs)), thus eliminating this effect.
However, restrictions on the control sample period in conventional back electromotive force sensors narrow the range of use for back electromotive force signals. Back electromotive force is proportional to speed. Therefore, when the signal quality is good, that is if a back electromotive force can be detected without any problem associated with the transient response caused by the differential item and without being affected by fast changes in current in a fast control sample period, then that back electromotive force can be applied in various types of control in systems other than the above-described load-unload system. Therefore, it is valuable. Other uses include the detection of abnormal operation in magnetic head positioning control systems that use back electromotive force monitoring during normal operation, and brake control in which back electromotive force is used when detecting abnormal operation.
However, the control sample periods in magnetic disk drives of recent years are 200 microseconds (xcexcs) at the fastest, and can go as low as 100 microseconds (xcexcs) or below. Therefore, conventional back electromotive force sensors with a sampling period of approximately 700 xcexcs could not be used as a speed signal during normal positioning operations.
With the foregoing in view, it is an object of the present invention to provide a back electromotive force sensor that can detect high quality back electromotive force signals, that is back electromotive force signals that can be detected without any problem associated with the transient response caused by the above differential item and without affecting fast changes in current in a fast control sample period.
To fulfill the foregoing object, the present invention provides a positioning device for driving and positioning an actuator using a VCM, with the following means for detecting the back electromotive force produced in the VCM coil: a first circuit that detects the terminal voltage of a VCM coil and outputs a band-limited signal that is a constant multiple of that value; a second circuit that detects the drive current for the voice coil motor and that outputs a voltage signal that is proportional to the above drive current; and a computation circuit that determines the difference between the output of the above first circuit and the output of the above second circuit. The first circuit has the effect of eliminating the item that is proportional to the VCM coil current differential, which was problematic in conventional methods.
Also, the present invention is configured so that the first circuit is a primary analog low pass filter with a time constant that is substantially equal to the ratio between the inductance and resistance of the VCM coil. This configuration simplifies detection and calculation of back electromotive force.
The band limiting is achieved in the first circuit by a primary digital low pass filter that is configured so that its time constant is substantially equal to the ratio between the inductance and resistance of the VCM coil. This configuration simplifies detection and calculation of back electromotive force.
In the low pass filter, the resistor that prescribes the filter gain and the capacitor that prescribes the filter time constant are located outside the IC chip. The second circuit and the computation circuit are provided as coefficient amplifiers in which operational amplifiers are used. The resistor that prescribes the gain of these coefficient amplifiers is located outside the IC chip. This configuration enables great scope for adjustment of filter and coefficient amplifier gain and for adjustment of the time constants for the above filters.
The magnetic disk drive that uses a VCM to drive an actuator with a magnetic head and that positions the head on the disk comprises: a first circuit that detects the terminal voltage of the VCM coil and outputs a band-limited signal that is a constant multiple of that value; a second circuit that detects the drive current of the voice coil motor and outputs a voltage signal that is proportional to the drive current; and a back electromotive force sensor that detects the back electromotive force in the VCM coil and consists of a computation that determines the difference between the output of the first circuit and the output of the second circuit. The actuator is provided with a speed control function from a feedback control system that uses the back electromotive force detected by the back electromotive force sensor. The sensor described above is used as a back electromotive force sensor. This configuration enables speed control of a magnetic head in a magnetic disk drive.
A VCM drive circuit that includes the present invention can be used to configure a back electromotive force sensor without the need for external operational amplifier circuits.